1. Field of the Invention
The present invention relates to an exposure information storage device for a photographic camera, and, more particularly, for such a storage device including an A-D converting means.
2. Description of the Prior Art
It is known in the prior art to electrically control exposure information, such as shutter and diaphragm information, in a photographic camera. For example, in the case of a timing circuit having an R-C time constant circuit, which is widely used for controlling the shutter time electronically, the closing operation of the shutter is effected as soon as the condenser is charged to a certain determined voltage. A photoconductive element, such as a CdS element is used as the resistance, R, or a photodiode with transistor is used equivalently, to supply information of the brightness of the object to be photographed. This type of exposure information control system requires that the light sensing element, such as a CdS element or photodiode, continues to receive the light beam from the object to be photographed while the shutter is in operation.
This conventional system cannot be employed, for example, in a single lens reflex camera in which the light sensing element receives the light beam through the lens. For such purpose, various storage devices have been proposed which enable exposure information such as brightness of the object to be electrically stored before taking a photograph. The exposure is set in accordance with this stored value.
Various kinds of exposure information storage devices have been proposed for such use. Some employ a condenser as a storage device. Others use a potentiometer to store the information by way of a mechanically controlled resistance element. Magnetic core devices are also used. These storage devices have the inherent deficiency that the storage operation is not fast enough. It is also difficult, in such devices, to store a very wide range of exposure information for a long time.
Considering the brightness of the object to be photographed as an exposure information, the light value range of the ordinary object is from LV-8 to LV+18, i.e., there are about 20 steps. The shutter speed and the aperture value also encompass a very wide range of values. The conventional storage device thus cannot react in a short time to store such a wide range of values.
Apart from the above mentioned storage devices, a storage device having an A-D converter has been proposed quite recently. In such device, a counter capable of high speed counting is used in the A-D converter. Thus, while the shortcomings of the conventional storage device can be eliminated, an erroneous signal is produced by the A-D converter when the converter is stepped up or down. Accordingly, in this device, an accurate storage operation cannot be carried out. To be more specific, the A-D converter usually contains a counter as its main part, which counter does not advance sufficiently quickly so that erroneous signals are transiently produced.
When the A-D converter is used as a storage device in a conventional way, the output of the light measuring circuit is converted from an analog value into a digital value so that the exposure is controlled based upon the number counted by the counter. It is necessary that the value counted by the counter be in accurate proportion to the output of the light measuring circuit. As a consequence, the value of the resistance in the ladder network used in the A-D converter must be designed with very high accuracy. Accordingly, the storage device becomes very expensive. This design also results in difficulties in the manufacturing of the converter which prevents its adoption in the exposure control circuit of the camera.